Fuel cell having perforated flow field

ABSTRACT

A fuel cell system includes a bipolar plate having a flow field formed therein. The flow field is partially defined by at least two adjacent channel portions separated by a wall portion. The wall portion includes a surface at least partially defining a passageway between the channel portions. The passageway may be sized so as to create a pressure difference between the channel portions. The pressure difference may draw at least a portion of a liquid droplet obstructing one of the channel portions toward and into the passageway.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 12/421,025,filed Apr. 9, 2009, the disclosure of which is hereby incorporated inits entirety by reference herein.

BACKGROUND

1. Field

The invention relates to fuel cells having perforated flow fields.

2. Discussion

A fuel cell is an electrochemical conversion device that produceselectricity from a fuel and oxidant that react in the presence of anelectrolyte.

Referring now to FIG. 1, a prior art fuel cell 10 includes a membraneelectrode assembly (MEA) 12 sandwiched between a pair of flow-fieldplates 14, 16. The MEA 12 includes a proton exchange membrane (PEM) 18and catalyst layers 20, 22 bonded to opposite sides of the PEM 18. TheMEA 12 further includes gas diffusion layers 24, 26 (anode, cathoderespectively) each in contact with one of the catalyst layers 20, 22. Asapparent to those of ordinary skill, the gas diffusion layer 24 andcatalyst layer 20 may collectively be referred to as an electrode.Likewise, the gas diffusion layer 26 and catalyst layer 22 may alsocollectively be referred to as an electrode.

The flow-field plate 14 includes at least one channel 28 n. As known inthe art, the at least one channel 28 n may form a spiral, “S,” or othershape on the face of the flow-field plate 14 adjacent to the anode 24.Hydrogen from a hydrogen source (not shown) flows through the at leastone channel 28 n to the anode 24. The catalyst 20 promotes theseparation of the hydrogen into protons and electrons. The protonsmigrate through the PEM 18. The electrons travel through an externalcircuit 30 to produce electrical power.

The flow-field plate 16 also includes at least one channel 32 n. Similarto the at least one channel 28 n, the at least one channel 32 n may forma spiral, “S,” or other shape on the face of the flow-field plate 16adjacent the cathode 26. Oxygen from an oxygen or air source (not shown)flows through the at least one channel 32 n and to the cathode 26. Thehydrogen protons that migrate through the PEM 18 combine with the oxygenand electrons returning from the external circuit 30 to form water andheat.

As apparent to those of ordinary skill any number of fuel cells 10 maybe combined to form a fuel cell stack (not shown). Increasing the numberof cells 10 in a stack increases the voltage output by the stack.Increasing the surface area of the cells 10 in contact with the MEA 12increases the current output by the stack.

SUMMARY

A fuel cell system includes a bipolar plate having a flow field formedtherein. The flow field is partially defined by at least two adjacentchannel portions separated by a wall portion. The wall portion includesa surface at least partially defining a passageway between the channelportions. The passageway is sized so as to create a pressure differencebetween the channel portions. The pressure difference draws at least aportion of a liquid droplet obstructing one of the channel portionstoward and into the passageway.

A fuel cell system includes a pair of flow-field plates and a membraneelectrode assembly sandwiched between the flow-field plates. At leastone of the flow-field plates includes a surface defining at least twoadjacent channel portions and a wall portion separating the channelportions. The channel portions are configured to deliver a fluid to themembrane electrode assembly. The wall portion has perforations thatcreate a pressure difference between the channel portions. The pressuredifference moves a liquid droplet obstructing one of the channelportions through the one channel portion.

An automotive fuel cell stack includes a plurality of fuel cells. Eachof the fuel cells includes at least one flow-field plate having a flowfield at least partially defined by two adjacent channel portionsseparated by a wall portion. The wall portion includes a surface atleast partially defining a passageway between the channel portions. Thepassageway is at least one of shaped, textured and coated so as tocreate a surface tension gradient between the surface at least partiallydefining the passageway and a surface of one the channel portionsobstructed by a liquid droplet. The surface tension gradient draws atleast a portion of the liquid droplet into the passageway.

While certain embodiments in accordance with the invention areillustrated and disclosed, such disclosure should not be construed tolimit the invention. It is anticipated that various modifications andalternative designs may be made without departing from the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, in cross-section, of a portion of a prior artfuel cell.

FIG. 2 is a plan view of a portion of a flow-field plate of FIG. 1.

FIGS. 3A and 3B are plan views of a portion of a flow-field plateaccording to an embodiment of the invention.

FIGS. 4A through 4C are plan views of a computational fluid dynamicmodel of a portion of another flow-field plate at several simulatedinstances of time.

FIG. 5 is an end view of a portion of a fuel cell according to anembodiment of the invention.

FIG. 6 is a side view, in cross-section, of the fuel cell of FIG. 5taken along line 6-6 of FIG. 5.

FIG. 7 is an end view of a portion of a fuel cell according to anotherembodiment of the invention.

FIG. 8 is a side view, in cross-section, of the fuel cell of FIG. 7taken along line 8-8 of FIG. 7.

DETAILED DESCRIPTION

Referring now to FIG. 2, the flow-field plate 16 includes severalparallel channels 32 n, i.e., 32 a, 32 b, 32 c. The channels 32 n areseparated by wall portions 34. In the illustration of FIG. 2, the flowof oxygen is indicated by arrow.

A droplet of water 36 has condensed and filled the entire cross-sectionof the channel 32 b thus obstructing the flow of oxygen downstream ofthe droplet 36. This flooding of the channel 32 b may affect thedurability of the fuel cell 10, may cause non-uniform distribution ofreactants to the channels 32 n, may cause non-uniform current generationby the fuel cell 10 and/or may affect the performance of the fuel cell10.

The flooding of channel 32 b may also promote flooding in the channels32 a, 32 c. The stagnant zone downstream of the droplet 36 may generatean increased electrical load for other active areas thus making watergeneration and flooding more probable in these otherwise active areas.This propagation of flooding may affect the operation of the fuel cell10.

Referring now to FIGS. 3A and 3B, an embodiment of a flow-field plate 38includes substantially parallel channels 40 n, i.e., 40 a, 40 b, 40 c.In other embodiments, a fewer or greater number of channels 40 n may beused. These channels 40 n may form a spiral, “S,” or other desired shapeon a face of the flow-field plate 38 configured to be in contact with amembrane electrode assembly (MEA) (not shown).

The channels 40 n share a common fluid source (not shown) and fluid sink(not shown) as known in the art. Oxygen, for example, may enter theflow-field plate 38 through an “in” port (not shown) which fluidlycommunicates with each of the channels 40 a, 40 b, 40 c. Water andun-reacted oxygen may exit the flow-field plate 38 though an “out” port(not shown) which also fluidly communicates with each of the channels 40a, 40 b, 40 c. Other configurations are also possible.

The channels 40 n are separated by wall portions 42 (current collectors,landing areas, etc.) The wall portions 42 include passageways 44 thatfluidly connect adjacent channels 40 n. In the embodiment of FIGS. 3Aand 3B, the passageways 44 are formed at regular intervals along thechannels 40 n. The passageways 44 may be formed at intervals from 10 to100 times the hydraulic diameter, D_(h), of one of the channels 40 n(where D_(h)=4×cross sectional area of the channel/perimeter of thechannel). In other embodiments, the passageways 44 may be formed atirregular intervals, staggered intervals, in an alternating pattern oras design considerations dictate. As explained below, the passageways 44are sized so as to draw condensed water from the channels 40 n into thepassageways 44 to reduce and/or prevent flooding of the channels 40 n.

As illustrated in FIG. 3A, a droplet of water 46 has condensed andfilled the entire cross-section of the channel 40 b. The droplet 46obstructs the flow of reactants (indicated by arrow), e.g., oxygen,hydrogen, air, etc., to the MEA (not shown) downstream of the droplet46. The passageways 44, however, allow the reactants to continue flowingthough the region of the channel 40 b downstream of the droplet 46. Thisprevents the generation of a static (stagnant) zone downstream of thedroplet 46. Due to lower pressures in the channels 40 a and 40 c, and 40b downstream of the droplet 46 (relative to a pressure in the channel 40b upstream of the droplet 46), the droplet 46 travels down the channel40 b until it encounters at least one of the passageways 44. The droplet46 is then drawn into these passageways 44.

As illustrated in FIG. 3B, the droplet 46 illustrated in FIG. 3A hasdispersed into smaller droplets 48 that may be carried with the flow ofoxygen and/or may vaporize by virtue of the high flow velocity in thechannels 40 n.

Referring now to FIGS. 4A through 4C, numbered elements of FIGS. 4Athrough 4C that differ by 100 relative to numbered elements of FIGS. 3Aand 3B have similar, although not necessarily identical, descriptions tothe numbered elements of FIGS. 3A and 3B. A computational fluid dynamicmodel of a flow-field plate 138 includes generally parallel channels 140n, i.e., 140 a, 140 b, 140 c, 140 d, separated by wall portions 142.Passageways 144 formed in the wall portions 142 fluidly connect adjacentchannels 140 n.

The passageways 144 are sized so as to create a pressure gradient thatwill drive water droplets in the channels 140 n toward and into thepassageways 144. For example, the passageways 144 may have a hydraulicradius of less than half of that for one of the channels 140 n (yet belarge enough to avoid issues related to surface tension and capillaryforces that may make purging and removing water droplets difficult.) Ifthe passageways 144 are too large (for example, large enough to promotecross-flow, i.e., uniform pressure, between the channels 140 n), such apressure gradient will not exist and water droplets may still form andstall within the channels 140 n.

Values of the parameters associated with the model are listed inTable 1. Of course, other values are also possible as dictated by designand/or performance considerations.

TABLE 1 Channel Height 787.4 microns Channel Width 1016 micronsPassageway Height 500 microns Passageway Width 500 microns Flow Rate1e−5 kilograms/second Contact Angle 110 degrees Droplet Thickness 1millimeter

As illustrated in FIG. 4A (time=0.024 sec.), the droplet 146 fills theentire cross section of the channel 140 c. The droplet 146 starts to bedrawn into two of the passageways 144 as illustrated in FIG. 4B(time=0.028 sec.) As illustrated in FIG. 4C (time=0.032 sec.), thedroplet 146 has been removed from the channel 140 c and resides withinthe two passageways 144 as droplets 148.

As apparent to those of ordinary skill, the mechanism by which thedroplet 146 enters the passageways 144 is governed by the transport ofmass, momentum, energy, charge and species through the fuel cellcomponents, such as a gas diffusion layer (not shown) and catalyst layer(not shown), and the channels 140 n.

The passageways 144 illustrated in FIGS. 4A through 4C are defined bystraight edges. In other embodiments, the edges that define thepassageways 144 may be curved or otherwise shaped as desired. Certainshapes, textures and/or coatings of the passageways 144 may create asurface tension gradient between, for example, a surface defining one ofthe passageways 144 if adjacent to the droplet 146 and a surface of thechannel 140 n in which the droplet 146 resides. As apparent to those ofordinary skill, design considerations and operating parameters of thefuel cell, such as temperature, flow velocity, etc., may influence theselection of the particular shape, texture and/or coating used.

The permeability of any porous components, such as the gas diffusionlayer (not shown) and catalyst layer (not shown), and/or the dropletinteraction with the surfaces (hydrophobicity/hydrophilicity) of thechannels 140 n and edges of the passageways 144 may have an effect onthe geometric design and effectiveness of the passageways 144 to removewater. Operating parameters, such as flow velocity, operatingtemperature, etc., may also have an effect on the geometric design andeffectiveness of the passageways 144 to remove water.

A full multiphase computational fluid dynamic or finite element studysimilar to that illustrated in FIGS. 4A through 4C (or experimentation)may be required to size/design the passageways 144. In lieu of such astudy, an analysis based on the Bernoulli equation may provide someinsight into droplet movement.

The general form of the Bernolli equation is:

${\frac{p_{1}}{\rho} + {\frac{1}{2}V_{1}^{2}} + {gz}_{1}} = {{\frac{p_{2}}{\rho} + {\frac{1}{2}V_{2}^{2}} + {gz}_{2}} = {const}}$

According to the above, with subscript 1 referring to quantities in theflooded channel 140 c downstream of the droplet 146 (which is stagnant,i.e., flow velocity V₁≈0, z₁=z₂), and subscript 2 referring toquantities in the channel 140 b or 140 d we have

$\frac{p_{1}}{\rho} = {{\frac{p_{2}}{\rho} + {\frac{1}{2}V_{2}^{2}}} = {{const\_}1}}$

Because V₂≠0 in order for this equality to hold true, we must have p₁>p₂which indicates the generation of a pressure gradient that pushes thedroplet 146 from the flooded channel 140 c to at least one of thepassageways 144.

Referring now to FIG. 5, numbered elements of FIG. 5 that differ by 200relative to numbered elements of FIGS. 3A and 3B have similar, althoughnot necessarily identical, descriptions to the numbered elements ofFIGS. 3A and 3B. A fuel cell 250 includes a corrugated flow-field plate238 having opposing face portions 252, 254, a contact plate 256 incontact with, and sealed against, the face portion 252 and an MEA 258 incontact with, and sealed against, the face portion 254. The corrugatedplate 238 and contact plate 256 define a plurality of channels 260though which a coolant, such as water, may flow. The corrugated plate238 and MEA 258 define a plurality of channels 240 through which a fuel,reactant, etc., may flow.

Referring now to FIG. 6, passageways 244 n, i.e., 244 a, 244 b, 244 c,are formed within the corrugated plate 238 and extend to the MEA 258.That is, in the embodiment of FIG. 6, the MEA 238 partially defines thepassageways 244 n. The passageway 244 a has a V-shape, the passageway244 b has a U-shape and the passageway 244 c has a polygonal shape. Ofcourse, the passageways 244 n may all have the same shape. Other shapesand positions are also possible.

Referring now to FIG. 7, numbered elements of FIG. 7 that differ by 300relative to numbered elements of FIGS. 3A and 3B have similar, althoughnot necessarily identical, descriptions to the numbered elements ofFIGS. 3A and 3B. A fuel cell 350 includes a flow-field plate 338 havingopposing face portions 352, 354, a cooling plate 356 in contact with,and sealed against, the face portion 352 and an MEA 358 in contact with,and sealed against, the face portion 354. A plurality of channels 360are formed within the cooling plate 356. The channels 360 deliver acoolant, such as water, to the face portion 352 to cool the flow-fieldplate 338. A plurality of channels 340 are formed within the flow-fieldplate 338. The channels 340 deliver a fuel, reactant, etc., to the MEA358.

Referring now to FIG. 8, passageways 344 n, i.e., 344 a, 344 b, 344 c,344 d are formed within the flow-field plate 338. The passageways 344 a,344 b, 344 c extend to the MEA 358. The passageway 344 d is formedcompletely within the flow-field plate 338. Other embodiments, such asthe embodiment of FIGS. 5 and 6, may also include passageways completelyformed within the flow-field plate. (If passageways are formed entirelywithin the corrugated plate 238 illustrated in FIG. 6, adjacent channels240 may be connected by small pipes or tubes to prevent the coolant fromleaking into the channels 240.)

The passageway 344 a has a V-shape, the passageway 344 b has a U-shape,the passageway 344 c has a polygonal shape and the passageway 344 d hasa round shape. Of course other shapes and positions are also possible.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. The words used in the specification arewords of description rather than limitation, and it is understood thatvarious changes may be made without departing from the spirit and scopeof the invention.

What is claimed is:
 1. A fuel cell comprising: a flow-field platedefining at least two adjacent channel portions and a wall portionseparating the channel portions, the wall portion having perforationstherein that are textured or coated such that surface tension gradientsbetween the channel portions and perforations draw liquid dropletsobstructing the channel portions into the perforations.
 2. The fuel cellof claim 1, wherein at least one of the perforations is completelyformed within the wall portion.
 3. The fuel cell of claim 1, whereineach of the channel portions has a hydraulic diameter and wherein aspacing of the perforations depends on the hydraulic diameter.
 4. A fuelcell comprising: a bipolar plate having a flow field partially definedby at least two adjacent channels separated by a wall that includes asurface at least partially defining a passageway between the channelsand being textured or coated such that a surface tension gradientbetween the surface and one of the channels draws a liquid dropletobstructing the one of the channels into the passageway.
 5. The fuelcell of claim 4 further comprising a membrane electrode assembly incontact with the bipolar plate and at least partially defining thepassageway.
 6. The fuel cell of claim 4, wherein the passageway iscompletely formed within the wall.
 7. The fuel cell of claim 4, whereinthe passageway is V-shaped, U-shaped, round or polygonal.
 8. The fuelcell of claim 4, wherein the passageway has a hydraulic radius and atleast one of the channels has a hydraulic radius and wherein thehydraulic radius of the passageway is less than one half the hydraulicradius of the at least one channel.
 9. The fuel cell of claim 4, whereina size of the passageway depends on at least one operating parameter ofthe fuel cell.